The present invention relates to mass memories or bulk storage units (BSU) of the metal nitride-oxide semiconductor field effect transistor (MNOS FET) type, and in particular, a BSU having over-write protection.
In the field of semiconductor memories, MNOS memories offer a number of advantages to a circuit designer. In particular, MNOS memories enable the design of a non-volatile solid state memory having a relatively fast access time. While generally these memories are used in a "read mostly mode", their inherent fast access time offers a temptation to the knowledgeable user of using the memory as a "scratch-pad memory" in certain applications.
Such scratch pad use presents a problem, though, in that it will prematurely fatigue the memory. Specifically, each memory cell of the BSU is typically comprised of two MNOS FET's that are coupled so as to exhibit a differential voltage threshold separation. The binary contents of each memory cell (i.e., "0" or "1") thus being dependent upon whether the cell exhibits a voltage in excess or below the threshold separation during the read operation. This threshold separation or "memory window", however, decays at an exponential rate with time and usage, such that each memory cell can typically only be written on the order of 10.sup.8 writes before its memory window has diminished to the point where the stored data cannot be detected accurately. Therefore, if a user were permitted to use an MNOS type BSU in a scratch pad fashion, he/she could potentially and prematurely fatigue all or a portion of the BSU.
In order to prevent premature fatigue, the present invention during each write protect interval, permits writing each address only once, via an access memory that flags the first writing of each address. Each subsequent attempt then producing a write abort. The present invention thus makes it very difficult for a user to operate the memory in a scratch pad fashion, since any use other than designed for would result in a write abort. The duration of the write-enable intervals also being within the discretion of the designer.
It is therefore a primary object of the present invention to prevent the over-writing of an MNOS type memory.
It is aother object of the present invention to prevent the over-writing via an access memory that permits only one write per protect interval.
These objects and others will become more apparent, however, upon a reading of the following description, as well as the "how to" for accomplishing these objectives.